Creating an ArrayList in Java? [duplicate]

Question

Possible Duplicate:
List versus ArrayList as reference type?

Whats the difference in creating an arraylist in the following ways?

List listA = new ArrayList(); and ArrayList alist = new ArrayList();

Answer 1

The first assigns a list to a variable defined by its interface, the second defines the variable by class.

The first declaration will let you change the implementation later:

List listA = new LinkedList();

is valid, while the second would not let you change the implementation:

ArrayList listA = new LinkedList(); // <<<=== INVALID

It is worth noting that starting with Java 5 List is a generic type, so you should specify the type parameter to improve type safety:

List<ClassA> listA = new ArrayList<ClassA>();